Variable sensitivity sound-powered receiver



Jan. 14, 1947. N. T. voLsK 2,414,129

VARIABLE SENSITIVITY SOUND-POWERED RECEIVER Filed March 5, 1945 INVENTOR. NICHOLAS T VOLSK ATTORNEY Patented Jan. 14, 1947 UNITED STATES PATENT OFFICE VARIABLE SENSITIVITY SOUND-POWERED RECEIVER (Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) 6 Claims.

The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to me of any royalty thereon.

This invention relates to sound-powered receivers, and particularly to a method and apparatus for varying their sensitivity which is often desirable when receivers of this type become connected to the telephone lines of different lengths.

It is well-known in the art that the sensitivity of the sound-powered receivers is superior to the sensitivity of the receivers of standard type, and, therefore, it is desirable to use the sound-powered receivers for extending the range of telephone communications. However, when receivers of this type become connected to the lines of relatively short length, the intensity of the signal impressed upon them is so high that the output of the receiver becomes too loud, and in extreme cases the receiver becomes over-loaded to such an extent that its articulation quality is impaired and the speech becomes unintelligible.

It is also known in the art to adjust the sensitivity of the sound-powered receiver by varying the impedance of the receivers circuit (U. S. Patent 2,344,338), this mode of adjusting the sensitivity of the receiver being accomplished by tapping the coil of the receiver, or inserting a series or shunt resistance in the circuit of the coil, and by equipping the handset with a switch which selects the desired sensitivity of the receiver, depending upon the line conditions. This type of sound-powered receiver is known in the art as the dual sensitivity receiver. The. dual sensitivity receivers do not accomplish the desired results for several reasons. As the term itself suggests, they have only two states of sensitivity which may be selected by the user by operating the two-position switch, and, as a consequence, cannot be adjusted to the optimum sensitivity when connected to the lines the impedance of which differs from the impedance of those two lines which can give optimum results at the maximum and minimum sensitivities that are made available in the receiver. The crudeness of the available adjustments may be appreciated more fully when one considers the fact that in one receiver of this type, which resort to the use of the tapped coil in the receiver, the number of efiective turns in the low sensitivity position of the switch is in the order of seventy-seven turns, while in the high sensitivity position it is in the order of 540 turns. No continuous adjustment of the ensitivity of the receiver is possible since, because of the minute dimensions of the receivers coil, tapping of the coil is a very difiicult operation. Moreover, even if such costly procedure were resorted to, a plurality of taps would call for a multiposition switch which would unduly complicate the structure, and increase the cost of the handset. Another serious disadvantage of any method now used in the art, which all resort to some method of varying the impedance of the receiver circuit, resides in the fact that any large change in the impedance of the receiver is undesirable from a system point of view. Thus, when the sensitivity of the receiver is varied by varying the number of turns included in the circuit, with 540 turns included in the circuit for high sensitivity and 77 for low sensitivity, there is a corresponding impedance change in the order of 50 to 1. The low impedance of the receiver when the latter is switched to the low sensitivity position will act as a short-circuiting device for all other receivers in a conference circuit connected to a short line; this is obviously inherently unsound and should be avoided by devising a sensitivity adjustment which would maintain the impedance of the receiver constant throughout its sensitivity range. Moreover, switching arrangements require additional wiring, periodic inspection and maintenance, and in some instances redesigning of handsets handle which raises the total cost of the set and creates multiplicity of standard parts.

The invention avoid-s these difiiculties by providing a lower reluctance pathwhen neededfor the flux produced by the permanent magnet of the sound-powered receiver, by means of a variable magnetic shunt which is capable of varying the reluctance of the magnetic circuit between the desired limits in a continuous manner. Accordingly, the adjustment of the sensitivity of the receiver i of the continuous type, and these limits may be arranged so as to shunt completely the air gap of the receiver (for all practical purposes), or to remove the shunting action altogether. The position of the magnetic shunt is selected so as to have no efiect on the impedance of the receiver itself, and, therefore, the impedance relationship between the outgoing impedance and the receiver remains undisturbed. As a matter of fact, there i a slight impedance compensation because of the favorable action of the magnetic shunt on the variable motional impedance. However, the value of the motional impedance per se is so insignificant as compared to the other impedances involved that from the point of .view of practical considerations it is proper to disregard it.

In placing the magnetic shunt in the structure of the sound-powered receiver, a caution must be exercised to place it so that it is preferably removed from the permanent magnet to avoid permanent ide-demagnetization, orreorientation of the magnetization of the permanent magnet, which obviously leads to the permanent loss of flux in the main working magnetic circuit of the permanent magnet.

The shunting action of the magnetic shunt from a purely mechanical point of view may be accomplished by means of a movable metallic plate made of ferromagnetic material, the posi-' tion of this plate with respect to the magnetic circuit of the receiver, and especially its air gaps, being adjusted by means of a shaft, thumb screw, a push lever, a cam or any other well-known mechanical movements which have. a control knob protruding from the case of the receiver, this knob being operated by a finger of the user of the set.

It is, therefore, an object of this invention to provide a sound-powered receiver with continuously variable sensitivity.

It is another object of the invention to provide a sound-powered receiver, the sensitivity of which can be varied with substantially no change in the impedance relationships of the circuits of which said receiver is a part.

It is another object of this invention to provide a sound-powered receiver with continuously variable sensitivity with the parameters of the substation circuit remaining substantially constant, irrespective of the state of sensitivity imparted to it.

It is still another object of this invention to provide a sound-powered receiver, the sensitivity of which may be controlled continuously from maximum to'minimum sensitivity bymeans of a magnetic shunt included in the magnetic circuit of the receiver. v

Still another object of this invention is to provide .a ound-powered receiver, the sensitivity of which'may be controlled with the parameters of the substation circuit remaining constant, thus allowing the line circuit to perform its function faithfully irrespective of the sensitivity setting of the receiver.

The novel features which are believed to be characteristic ofthisinvention are set forth with particularity in theappende'd, claims. The invntion itself, however, both as to its organization and method of'operation, together with the further objects and advantages thereof, may best be understood by reference to the following description in connection with the accompanying drawing in. which: V r

"Figure 1 is a side elevational view of a soundpowered receiver provided with a magnetic shunt;

Figure. 2 is a. vertical cross-sectional view of the receiver of Fig. '1 taken along'line 2-2;

Figure 3 is a'plan view of the receiver illustrated'in Fig. 1.;

, Figure l'is'aside elevational view of 'a modified formof the magnetic shunt;

'Figure5 isthesideview'taken along line E--5 of 'the 'magnetic shunt illustrated in Fig. 4;

v Figure ofisa. side View of a magnetic shunt similar. to that'illustrated in Figure 1 with a modified mechanical linkage; 'Figure 'Iis a topplan view of the mechanical linkagein Fig. 6;. U.

Figure 8 is a perspective viewof thedual sensitivity receiver mounted in ahandset c'asi'ng'with 4 the sensitivity control knob attached in the casing as illustrated in the figure, and

Figure 9 is the schematic diagram of a substation circuitwhich may be used in connection with the sound-powered receivers disclosed in the Figs. 1 through 8.

Referring to Fig. 1, it illustrates a well-known sound-powered receiver, which includes the modifications proposed by the principal invention, A permanent magnet Ill is mounted between a ferromagnetic disc l2 and a ferromagnetic plate I l, the three being held together by means of bolts it; similar mechanical coupling between plate i4 and disc i2 is provided on the other'side by a nonmagnetic spacer iii. The nonmagnetic spacer is attached to disc l2 and plate i l by set screws !3 and i5 on top, and similar set screws on the bottom, the bottom screws being not visible in the figures. Two U-shaped ferromagneticmembers 2!] and 22 are securely attached to plate 51 and disc l2 respectively by set screws i? and [9, the bottom set screws being not visible in the figures. The U-shaped members are so proportioned as to form two slit-like air gaps 24 and 25 between them, the view ofthe left air gap 25 in Fig. 1 being partially obliterated by a shaft 26 which is connected to a ferromagnetic plate 28 through a cam block 33. Only the right portion of an armature 32 is visible in Fig. 1, this end of the armature being attached to a connecting rod 3% which connects the armature with a'corrugate'd diaphragm 36 of the receiver. The rod is fixed to the armature by means of two nuts 38 and 40 on one side and to the diaphragm by a riveted or welded connection 42 on the other side. The armature represents a flat leaf made of ferromagnetic material which is centrally positioned in the air gaps formed by the U-shaped members, and is attached on its left side by a set screw 44 to the nonmagnetic spacer N3, the upper portion of which is U-shaped for facilitating this connection. The sound-coil of the receiver is wound on a bakelite frame 46, this frame being so dimensioned that it forms a sliding fit with the U- shaped members 2!! and 22. The turns of the wire wound on frame 48 are not visible in Figure 1 since their View is obliterated completely by magnetic shunt plate 28, but they are visible at 55 in Fig. 5. The two terminals of the coil are illustrated in Fig. 2' at E3 and 59; these are connected across the anti-side-tone portion of the substation circuit.

The functioning of the sound-powered receiver illustrated in the Figs. 1, 2 and 3 is very wellknown in the art'and will be mentioned briefly here only to coordinate the functioning of the magnetic shunt and of the receiver. When varying direct current is impressed on sound coil 58, of the receiver, it produces a comparatively strong flux through the center of the coil, and since the armature of the receiver is positioned in the center of the coil the state of its magnetism will follow thestate of flux existing in the center'of the coil. Since at the air gaps the armature is in a strong magnetic field created by thepermanent'magnet, the armature will be either attracted, or repelled by the, U-shaped magnetsthus transmuting the electrical undulations into mechanical vibrations which are transmitted to diaphragm '36 of the receiver. Since one of the'factors controlling the'sensitivity of the receiver is the density of the permanent flux in the fair gapsf'the sensitivity of the receiver'may'be controlled by decreasing this density to any desired lower value. If the magnetic shunt is positioned properly with respect to the permanent magnet as well as the sound coil, it-will have no permanent demagnetizing effect on the permanent magnet, nor will it have any significant efiect on the impedance of the sound coil of the receiver. With the magnetic shunt thus positioned in the receiver, the impedance looking into the substation circuit will not be affected by any adjustment of magnetic shunt and, as a consequence, the functioning of the line circuit and the state of the properly adjusted impedances in the substation circuit remains unaffected by any adjustment of the sensitivity of the receiver. 7

This is accomplished in Fig. l by placing magnetic shunt-plate 28 adjacent to the U-shaped members and 22, this shunt providing a low reluctance path for the flux produced by the permanent magnet when the magnetic shunt is moved into the position illustrated in the Figs. 2 and 3 where it makes direct contact-with its upper portion-with plate I 4 and member 26, andwith its lower portion-with member 22 and disc I 2. In this position the magnetic shunt produces magnetic short circuit across the air gaps of the receiver and, therefore by far the largest part of the flux generated by the permanent magnet ID will follow the path of least resistance, which is obviously the magnetic shunt 28. When this is the case, the sensitivity of the receiver is reduced practically to zero; such position of the magnetic shunt will be desirable only when it is necessary to protect the receiver from high level sounds, such as gunfire. The sensitivity of the receiver may be adjusted by varying the lengths of the air gaps so as to obtain the desired reluctance ratio between the working magnetic circuit of the receiver and the parasitic magnetic path provided by the magnetic shunt. Since mechanical movements are available which can change the position of the magnetic shunt with respect to the main magnetic circuit with any desired degree of precision, it is possible to provide smooth and continuous control of the sensitivity of the receiver between the available limits.

The mechanical control illustrated in the Figs. 1, Zand 3 consists of a cam assembly including a cam block provided with a circular hole and a cam 52, Fig. 3, forming a sliding fit with the hole. The cam itself is mounted on a shaft 54, connected through a threaded engagement 56 to the external shaft 26. Shaft 26 extends outwardly through all the casings of the receiver and of the handset where it terminates in a knurled knob 80!] mounted adjacent to the outer surface of the receivers casing 802. By turning knob B00 either in one direction or the other the sensitivity of the receiver may be either decreased or increased until the optimum degree of transmutation is obtained with the line conditions prevailing at any given moment. The maximum sensitivity of the receiver is obtained when the magnetic shunt is in the extreme upper left position with respect to the main magnetic circuit, as illustrated by the dotted lines in Fig. 3, the minimum sensitivity being obviously that illustrated by the solid lines in the same figure.

It is to be noted that frame 46 of the sound coil 50 extends outwardly at 6B and 62 on the side adjacent to the magnetic shunt extensions 60 and B2 are used as bearing surfaces for shaft 54.

The receiver is assembled in a conventional manner, and assembling of the magnetic shunt is accomplished by slipping cam 52 into the hole in block 30, and by force-fitting shaft 54 into the hole in the cam. The receiver is then mounted in the Bakelite casing and the casing is fitted into the outer shell 862 provided with a small opening for shaft 26. Shaft26 is inserted into the hole and screwed tightly against the inner portion 54 of the shaft until it forms a tight joint 56 with the latter, which completes assembling of the receiver.

Figs. 4 and 5 illustrate a modified mounting of shunt 400; the cam block 30 in this case is connected to that portion of the shunt which is to the right of plate 62. Either in the Figs. 1 to 3 inclusive or 4 and 5 any sudden shock imparted to the external knob 800, which is incidentally mounted in a favorable position on the curved surface of the shell, will not be transmitted to the sensitive parts of the receiver but will be stopped by the engagement of knob 800 against the casing, since cam 52 is free to slide laterally in the cam block 30.

Figs. 6 and 7 illustrate a modified linkage mechanism which may be used between the magnetic shunt 28 and shaft 54. It consists of a wire I00 bent so as to form a wedge-like surface with respect to the shunt, and the shunt itself is provided with an eye I02. It is apparent from the examination of Fig. 7 that the sliding movement of shaft 54, as indicated by the arrows, will result in shifting the magnetic shunt toward or away from the magnetic plate l4, thus producing the same final result as shunt 28 in the Figs. 1 to 3 inclusive. In the embodiment illustrated in the Figs. 6 and 7, shunt 28 should form a sliding fit with the bottom magnetic disc l2 so that there is a continuous magnetic short circuit between the shunt plate 28 and the disc. In all arrangements the extensions 60 and 62 are provided with the guiding surfaces preventing tilting of the shunt.

Referring now to Fig. 9, it discloses one type of substation circuit which may be used in connection with the sound-powered receiver. The circuit disclosed in Fig. 9 is widely known in the U. S. telephone industry as U. S. Army Telephone Set EE-S, and, therefore, requires no detailed description. It is included in the disclosure to emphasize some of the impedance relationships, the maintenance of which is very desirable, if not imperative, when the sensitivity of the receiver is varied between its maximum and minimum limits. The sub-station circuit includes a hand generator 909, which is used for ringing purposes, whose cranking handle is coupled through a cam to an armature 902, so as to transfer this armature to a contact 994 at the beginning of the ringin operation by the local operator. Armature 902 normally rests on contact 906 which is connected to a ringer 908, the circuit of which is completed through a condenser 9l0. The outgoing telephone line conductors M2 and 914 are connected across a conventional induction coil of the sub-station circuit which includes three windings 9l6, 9|8 and 920, the upper winding being connected to the outgoing line conductor 9l2 while the lower winding 920 is connected to the outgoing conductor 9l4 through condensers 922 and 924. A carbontype transmitter $26 is connected across the primary winding 918 of the induction coil through a local source of potential 928 and a handset switch 930, thus forming a local battery substation circuit. The sound-powered receiver is shown at 932 and it corresponds to the soundpowered receivers illustrated in Figs. 1 to 8 inclusive. The receiver is connected across the secondary coil 920 and condenser 922, this coilcondenser combination formin a well-known anti-side-tone. portion of the sub-station circuit. It has .been previously mentioned in the specification that ,for maximum efiiciency during reception the impedance of receiver .932 must match the impedance of the circuit across which it is connected, or stated differently, it must 'match the impedance of the circuit looking into the line. .when it is measured across the junction points 936 and 931. In the. prior art all methods of adjusting the sensitivity of receiver 932 used some means for varyin the impedance of the receiver circuit, either by tapping coil 938 of the receiver, inserting a series resistance with this coil; or shunting it by means of a shunt resistance. With the tapped coil arrangement, as mentioned before, the impedance of the receiver is reduced in the ratio of 50 to l, i. e. at high sensitivity the impedance of the receiver is fifty times higher than at low sensitivity. Such large change in impedance acts, for practical purposes, as a short-circuit for all other receivers which may be connected to the receiving end of the line in a conference call which prevents all other receivers from participating in the conversation. Thus adjusting of the sensitivity of one receiver varies the sensitivity of all other receivers. Moreover, since during transmission the approximate balanced condition of the alternating current bridge formed by the substation circuit and the line is obtainable only with the characteristic line impedance, the bridge will be more unbalanced when the line impedance is other than the characteristic impedance, and as a consequence, with the low impedance'receiver connected across the bridge, the side-tone signal through the receiver will be higher.

The magnetic shunt principle used for adjusting the sensitivity of the receiver is illustrated in Fig. 9 by the permanent magnet H), the ferro magnetic plate It, disc [2,. and the adjustable magnetic shunt 2B which may be shifted to a suficiently close position with respect to plate l4 and disc I2 so as to shunt in part the air gap 24 as described previously in connection with Figs. 1 to 8 inclusive. This adjustment of the position of the shunt element 28 does not alter the impedance of the receiver, and, therefore, the difiiculties. outlined above are avoided; this is true throughout the range of the sensitivity of the receiver because the magnetic shunt 28 is positioned so that its plane is parallel to the magnetic axis of the coil. Since the coil is surrounded by the U-shaped members 28 and 22, it has a magnetic path of its own of very low reluctance, and, therefore, only an insignificant amount of leakage flux will reach the magnetic path offered by the magnetic shunt 28. Thus the. impedance of the receiver is not afiected by the shunt irrespective of its position.

In the sound-powered unit illustrated in Figs. 1 to 8 inclusive, another equally advantageous position may be found for placing the magnetic shunt, and it is the position occupied by the nonmagnetic spacer H3. The mechanical arrangements illustrated in the drawings for shifting the position of the shunt may be applied to the magnetic shunt when it is placed in the plane of'the non-magnetic spacer I 8.

The invention has been described in connection with the sound-power unit which. has been the receiving as well as the transmitting units.

While the invention has been described with reference to several particular embodiments, it will be understood that various modifications of the apparatus shown may be made within the scope of the following claims.

I claim:

1. A sound-powered receiver including a permanent magnet, a sound coil, a working magnetic circuit for said magnet and said coil, and an adjustable magnetic shunt making said receiver a continuously variable sensitivity receiver, said shunt being positioned with respect to said circuit so as to maintain the impedance of said coil substantially constant through the sensitivity range of said receiver.

2. A sound-powered telephone unit including a permanent magnet, a working magnetic circuit connected to said magnet, a magnetic shunt mounted in said unit, and mechanical instrumentalities for varying the position of said shunt with respect to said magnetic circuit for varying the sensitivity of said unit with the impedance of said receiver remaining substantially constant.

3. A sound-powered receiver including a sound coil, a permanent magnet, a working magnetic circuit connected to said magnet, and a variable magnetic shunt for varyingthe reluctance of said magnetic circuit, thereby varying the sensitivity of said receiver, said magnetic shunt being so positioned with respect to said sound coil as to maintain the impedance of said coil substantially constant while the position of said shunt is varied between its operating limits.

4. A telephone substation circuit including a sound-powered unit, and magnetic instrumentalities for varying the sensitivity of said unit with the impedances of said. substation circuit remaining constant over the range of sensitivity of said unit while the. position of said instrumentalities is varied between the operating limits.

5. A telephone sub-station circuit including an anti-side-tone circuit, a variable sensitivity sound-powered receiver connected to said antiside-tone circuit, a sound coil in said receiver, the impedance of said coil having a predetermined relationship to the impedance of said antiside-tone circuit, and adjustable magnetic instrumentalities within said receiver for varying the sensitivity of said receiver with said relationship of said impedances remaining substantially. 

